Process for the flocculation of photographic gelatino silver halide emulsions

ABSTRACT

A PROCESS FOR FLOCCULATING A PHOTOGRAPHIC GELATION SILVER HALIDE EMULSION BY ADDING A FLOCCULATING AGENT TO THE EMULSION TO INITIATE FLOCULLATION AT PH 3.0-4.2, ADDING WASH WATER TO THE EMULSION WHILE MAINTAINING THE PH AT 3.0-4.2, AND SUBJECTING THE MIXTURE TO A MILD CENTRIFUGAL SEPARATION AT 5*C.-25*C. TO PROVIDE AN EMULSION DIRECTLY SUITABLE FOR REDISPERSION WITHOUT STRONG ADHESION OF THE EMULSION TO THE WALL OF THE SEPARATOR.

1972 MOTOHIRO UJIHARA ETAL 3,707,378

PROCESS FOR THE FLOCCULATION 0F PHOTOGRAPHIC GELATINO SILVER HALIDE EMULSIONS Filed May 14, 1971 FIG I SPECIFIC ELECTRIC CONDUCTIVITY (AU/cm) RATIO OF WATER T0 0m: PART OF EMULSION SPECIFIC ELECTRIC CONDUCTIVITY (MU/cm) I I I I 0 I 2 3 4 5 RATIO OF WATER TO ONE PART OF EMULSION INVENTOR MOTOHIRO UJIHARA IISUO FUJII SWJLAQQIWFKM, Ml

ZIHK qf ATTORNEYS United States Patent O 3,707,378 PROCESS FOR THE FLOCCULATION OF PHOTOGRAPHIC GELATINO SILVER HALIDE EMULSIONS Motohiro Ujihara and Itsuo Fujii, Kanagawa, Japan, assignors to Fuji Photo Film Co., Ltd., Ashigara-shi, Kanagawa, Japan Continuation-impart of abandoned application Ser. No. 643,349, June 5, 1967. This application May 14, 1971, Ser. No. 143,341

Int. Cl. G03c 1/02 US. Cl. 96-114.8 11 Claims ABSTRACT OF THE DISCLOSURE A process for flocculating a photographic gelatino silver halide emulsion by adding a flocculating agent to the emulsion to initiate flocculation at pH 3.04.2, adding wash water to the emulsion while maintaining the pH at 3.0-4.2, and subjecting the mixture to a mild centrifugal separation at 5 C.25 C. to provide an emulsion directly suitable for redispersion without strong adhesion of the emulsion to the wall of the separator.

This invention is a continuation-in-part application Ser. No. 643,349, filed June 5, 1967, now abandoned.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a process for the flocculation of a photographic gelatino silver halide emulsion and more particularly to a process for desalting and dehydrating a gelatino silver halide emulsion during the flocculation thereof.

Description of the prior art In the manufacture of silver halide emulsions, the preparation thereof is preferably conducted in as short a time as possible. For this purpose, the addition of silver salt and halide to binder solution, physical ripening, desalting and dehydrating step must be continuously conducted. However, each step has been independently conducted because the desalting and dehydration steps have been the methods hereafter described. Thus, a continuous process for the preparation of silver halide emulsion depends on the desalting and dehydration steps.

In the past, excessive salts in a photographic gelatino silver halide emulsion have been removed by supplementally adding a predetermined amount of aqueous gelatin solution to a physically ripened emulsion, solidifying the emulsion by cooling, noodling, and then washing with water. However, thi process has the disadvantages that the desalting efliciency is low, a long period of time is required for conducting the process, a large apparatus is required, a high cost is required for its installation and the system must be precisely controlled for obtaining emulsions having homogenous qualities. Moreover, this process does not achieve dehydration.

There is also known a process in which desalting and dehydration of a photographic emulsion is achieved by allowing a physically ripened gelatino silver halide emulsion containing a suitable flocculating agent to stand and flocculate resulting in the flocculation of silver halide particles together with gelatin and the removing the thus formed supernatant liquid by decantation. Recently, various effective flocculating agents have been found and hence, the latter process has been utilized commercially for simplifying the desalting and dehydrating process. On comparing with the above-mentioned desalting process,

the latter process has the advantages that it requires no large scale installation and the time required for conducting the desalting and dehydrating process is markedly reduced. However, since the precipitation of the aggregates is achieved by natural settling through gravity, there is required a certain settling time to enable suflicient decantation of the supernatant liquid necessary for desalting and dehydration. When practicing this process on an industrial scale, the settling procedure requires considerable time. Furthermore, there is a limit to the extent of desalting which can be achieved by only one flocculation step followed by decantation. Hence, in order to obtain suflicient desalting there is usually required a serie of steps of washing the precipitates with fresh water, re settling, and redecanting. Moreover, in cases where the pH of the system must be adjusted to effect flocculation, a troublesome pH adjustment is required for each settling step in the repeated procedure.

Methods of separating a dispersion of solid fine particles into a solid phase and liquid phase by centrifugal force are well known and have been frequently applied in industry. Moreover, it is possible, theoretically, to separate the silver halide particles in a photographic gelatino silver halide emulsion by centrifugal force. Indeed, there ha-ve been attempts to apply such a technique to the desalting of photographic emulsions. However, since the silver halide particles contained in a physically ripened photographic gelatino silver halide emulsion are of an extremely small size, e.g., on the order of :submicron to micron size, and as the emulsion is an extremely stable dispersion in which gelatin is present as a protective colloid, it requires a very large centrifugal force and a long separation time to conduct the complete solid-liquid separation of the emulsion.

Further, since thus separated silver halide particles are subjected to a large centrifugal effect, they are strongly attached to the inside wall of the separator and very strongly bonded to each other. Hence, it becomes very difficult to redisperse the silver halide particles in a subsequent step.

BRIEF DESCRIPTION OF THE INVENTION Therefore, it is an object of this invention to provide a process for the improved flocculation of photographic silyer halide emulsions which is not accompanied by the above mentioned drawbacks.

Another object of the present invention is to provide a process for obtaining a dense gel-form photographic emulsion which can be easily treated in a reduced period of time.

Still another object of this invention is to provide a process for desalting and dehydrating photographic gelatino silver halide emulsions utilizing centrifugal force with flocculation which is unaccompanied by the abovementioned drawbacks.

Those objects can be attained according to the present invention by subjecting to a comparatively low centrifugal force at a comparatively low temperature a photographic gelatino silver halide emulsion, at a pH of 3.0 to 4.2, in which silver halide particles and gelatin are flocculated by the addition of a flocculating agent, together with wash water in a proportion suitable for reducing the salt content to a desired value and conducting the flocculation, the desalting and the dehydration simultaneously by centrifugal separation.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are plots of specific electrical conductivity versus the ratio of water to one part of emulsion based upon Example 5.

3 DETAILED DESCRIPTION OF THE INVENTION Into a photographic gelatino silver halide emulsion which has been physically ripened according to conventional methods there is added a flocculating agent in an amount necessary for flocculating silver halide particles with gelatin and, e.g., the pH of the emulsion is adjusted to initiate the flocculation of silver halide particles with gelatin. The emulsion is continuously withdrawn at a constant flow rate from this pre-treating zone, while stirring slowly, and continuously mixed with wash water at a constant flow rate. The resulting mixture is supplied at a comparatively low temperature of 5 C. to 25 C. into a centrifugal separator having a comparatively low rotational number, i.e., having a comparatively low centrifugal effect of G to 1000 G, to cause solid-liquid separation and to provide the concentrated gelatino silver halide emulsion.

It is to be understood that in accordance with the present invention the pH of the emulsion must be in the range of 3.0-4.2 at the initiation of flocculation, i.e., the pH is either adjusted to 3.0-4.2 prior to adding the flocculating agent, or simultaneously with adding the flocculating agent as in the examples where in a continuous process the flocculating agent is added in the mixing zone with, e.g., an acid. Of course, this does not exclude instances where pH adjustment to 3.042 occurs after flocculating agent addition to initiate flocculation as in this instance the critical criterion that the pH be 3.04.2 at the initiation of flocculation would still be met. For instance, this last embodiment occurs where the pH of the emulsion is not adjusted during flocculating agent addition per se but,

e.g., where the pH of the Wash water causes the pH of the emulsion to be in the range 3.0-4.2 whereby flocculation is initiated.

In all instances, centrifugal separation is at a pH of 3.0 to 4.2.

In the process of the present invention, any known method may be utilized to cause the flocculation at a pH of 3.0-4.2 of silver halide particles and gelatin in gelatino silver halide emulsions. For example, the flocculation of silver halide and gelatin may be salted out by adding an inorganic salt such as Glaubers salt into a photographic gelatino silver halide emulsion prepared by a conventional manner, or by adding thereto compounds such as are disclosed in US. Pat. Nos. 3,482,980 and 3,552,053. Another method comprises mixing the emulsion with a high molecular weight polymer capable of forming a hydrophobic complex compound with gelatin, such as a condensation product of sodium naphthalene sulfonate and formaldehyde or sodium polystyrene sulfonate, at a pH lower than the isoelectric point of gelatin to flocculate the silver halide and the gelatin. Also, the photographic gelatino silver halide may be prepared by using a gelatin derivative capable of being flocculated under an acid condition, such as phthalated gelatin or carboxymethylated gelatin. In

this case, the silver halide particles and the gelatin may be flocculated by reducing the pH of the emulsion to pH 3.0 to 4.2.

Most preferably the gelatin concentration of the silver halide emulsion before adding the flocculating agent is less than 4 wt. percent.

The ratio of wash water to emulsion to be used for desalting is selected so that sufiicient desalting is achieved whereby a photographic emulsion is eventually obtained which may be redispersed in a necessary amount of an aqueous gelatin solution. That is, as the amount of wash water is increased, the desalting degree of the finally obtained emulsion becomes lowered. Accordingly, it is not necessary to limit the range for wash water/emulsion. However, in light of the fact that as the amount of wash water is increased, separating water becomes more complicated and the scale of the apparatus becomes larger, it is preferable, from practical point of view, to add wash water in an amount less than 10 times the amount of emulsion. The wash water is adjusted to substantially the same pH as the silver halide emulsion and then is added thereto, i.e., the water must not change the pH of the emulsion/water so it is outside the range 3.0-4.2.

The temperature of the system is selected so that the condensate achieves a non-sticky gelled state due to the gelatin contained therein. Suitable temperatures of the mixture of the emulsion and the wash solution range 5 C. to 25 C.

The centrifugal separator is operated under such conditions that no problems arise when it is attempted to separate the condensate and redisperse it and a maximum amount of the emulsion mixture capable of being separated is produced. Since the emulsion has been preliminarily flocculated a rapid solid-liquid separation is possible in a period of 1-2 minutes by the above-mentioned comparatively low centrifugal force and so as to permit redispersing the thus separated condensate in a gelatin solution, a centrifugal effect of less than 500 G is most suitable.

If it is necessary to adjust the pH of the photographic emulsion to initiate flocculation the pH of the wash water to be mixed with the emulsion may be adjusted to have substantially the same value as the pH of the emulsion to prevent alteration of the adjusted pH of the emulsion. The pH of the wash water and that of the emulsion are substantially identical because when the pH of the wash water is greatly different from that of the emulsion the pH of the emulsion which is going to precipitate changes, and precipitation may incompletely occur or precipitation may not occur at all. Therefore, the pH of the wash water should be within the range which does not change pH of the emulsion so as to hinder precipitation, i.e., so the pH of the emulsion is not changed to outside the range 3.0 to 4.2. Needless to say, it is most preferable, once the pH of the unwashed emulsion is set, to make the pH of the wash water as close as possible as that of the emulsion so that no significant pH alternation of the emulsion occurs. This is the practice followed in Examples 1-3.

Or, without preliminarily adjusting the pH of the emulsion, the pH of washing water may be adjusted to cause flocculation of the emulsion when the emulsion is mixed with the wash water in a constant ratio to bring the pH of the mixture to the flocculation range. In this instance, of course, the pH of the wash water must still be such that the pH of the emulsion is not caused to be outside the 3.0-42 range. Alternatively, a mixture of a photographic emulsion and wash water may be mixed with a constant amount of a pH adjusting agent to cause flocculation and the resulting emulsion may be introduced into a centrifugal separator.

If a large quantity of wash water is not necessary, a photographic emulsion is preliminarily mixed with the requisite amount of washing water in the emulsion preparing bath and, after initiating flocculation, the mixture is introduced into a centrifugal separator. Moreover, if wash water is not necessary after initiating flocculation the emulsion may be directly introduced into a centrifugal separator. Wash water is, however, still used, although in lesser amounts.

The centrifugally separated gel-form concentrated emulsion may be withdrawn in a batch manner from the centrifugal separator after accumulation therein, or it may be continuously withdrawn during the centrifugal separation. Any known centrifugal separator may be employed in the practice of the invention.

Since the photographic gelatino silver halide emulsion to be treated by the process of this invention is in a state where the silver halide particles and the gelatin in the emulsion are flocculated by the addition of a proper flocculating agent at a, or to provide a, pH of 3.0-4.2, the rapid solid-liquid separation of the emulsion is possible by a comparatively low centrifugal effect and since the removal of water from the emulsion is sufliciently effected by the centrifugal force, the density of solid in the condensate of the silver halide emulsion is very high and at the same time a high salting effect can be obtained.

Also, since the thus obtained condensate is in a consolidated gel-form state due to the gelatin contained therein and caused by cooling and as the centrifugal effect is low, bonding of the silver halide particles to each other is limited. Moreover, the silver halide particles are not strongly attached to the wall of separator and may be separated easily from the wall and may be easily dispersed in water or an aqueous gelatin solution by heating. Alternatively, the thus obtained condensates, that is, the gelled dense emulsion, are in a state suitable for preserving by cooling as they are produced.

As mentioned above, by properly selecting the conditions, a gelled dense emulsion having a desired salt con tent and which can be easily treated may be rapidly prepared. According to the process of the present invention the desalting and dehydration process can be accomplished in only one solid-liquid separation operation and the time of the process can be reduced to /2 to A of that in a conventional natural settling method. Moreover, the concentration of solid in the thus obtained dense emulsions can be increased to 2-4 times as large as that of the conventional methods.

The process of this invention is illustrated by the following examples.

EXAMPLE 1 A silver bromide emulsion containing 55 parts by weight of silver-bromide and 5 parts by weight of dried gelatin per 1000 parts by weight of the total emulsion was prepared by a conventional manner. After finishing the physical ripening, the emulsion was mixed at a temperature of 40 C. with sodium polyethylene sulfonate as a flocculating agent in an amount of by weight of the gelatin in the emulsion and the pH of the resulting emulsion was adjusted from 6.0 to 4.2 by adding salicylic acid. The thus prepared silver halide emulsion in which the silver halide particles were in a state of becoming flocculated with the gelatin, had a specific electric conductivity of 56,000 ,uV./CII1. While stirring the system, the emulsion was withdrawn in a constant rate and continuously mixed with water of 16 C. the pH of which has been adjusted to 4.2 in a mixing ratio of l to 2 emulsion to water. In this case the electric conductivity was 20,000 v./cm. The resulting mixture was then continuously supplied into a centrifugal separator rotating at a centrifugal effect of 500 G at a flow rate sufficient to maintain an average retention time of 2 minutes to conduct solid-liquid separation. The thus obtained gelled dense emulsion was not sticky and the separation thereof from the wall of the centrifugal separator was easily accomplished. The dense emulsion was redispersed in warm water of 60 C. and after adjusting the pH of the emulsion to 6.2, the emulsion was mixed with dried gelatin in an amount of 85 parts by weight per 1000 parts by weight of the emulsion followed by ripening. The specific electric conductivity of thus obtained photographic silver halide emulsion was 1,700 ,uv./cm. The thus obtained emulsion has the same properties as a photographic gelatino silver halide emulsion prepared by a conventional process.

On the contrary, the photographic silver halide emulsion, which was prepared in the same manner except that water was not added to the emulsion, had a specific electric conductivity of 3,400 v./cm.

EXAMPLE 2 A silver chlorobromide emulsion containing 30 parts by weight of silver bromide, 60 parts by weight of silver chloride and 5 parts by weight of dried gelatin per 1000 parts by weight of the total emulsion was prepared by a conventional manner and mixed with a fiocculating agent in an amount of 10% by weight of gelatin in the emulsion and the pH of the resulting emulsion was adjusted to pH halide emulsion having a specific electric conductivity of 75,000 ].LV./ cm. and in a state of becoming flocculated with the silver halide and the gelatin Was continuously mixed with washing water having the pH of 3.8 at 10 C. in a mixing ratio of 1 to 3.3 whereby the specific electric conductivity was reduced to 19,000 ,uv./cm. The resulting mixture was continuously supplied into a centrifugal separator having a centrifugal effect of 150 G at a flow rate necessary to sustain a retention time of 1.5 minutes to conduct solid-liquid separation, whereby a gelled dense emulsion in the state of that produced in Example 1 was obtained. By adding the emulsion together with gelatin into warm water and ripening as in Example 1, a photographic gelatino silver halide emulsion having a specific electric conductivity of 1,700 /1.V./CII1. was obtained. The emulsion showed the same properties as a photographic gelatino silver halide emulsion prepared by a conventional process.

On the contrary, the photographic silver halide emulsion, which was prepared in the same manner except that water was not added to the emulsion, had the specific electric conductivity of 4,400 ILV./Cm.

EXAMPLE 3 A silver halide emulsion containing 60 parts by weight of dried gelatin per parts by weight of the total emulsion was prepared by a conventional manner and mixed with a flocculating agent as in Example 1 in an amount of 4% by weight of the gelatin in the emulsion and the pH of the resulting emulsion was adjusted from 5.3 to 3.0 by adding hydrochloric acid. The thus obtained silver halide emulsion with a specific electric conductivity 47,000 ,uv./cm. and in a state of becoming fiocculated with the silver halide and the gelatin was continuously mixed with wash water having the pH of 3.0 at 25 C. in a mixing ratio of 1 to 3.5, whereby the specific electric conductivity was 18,000 ,uv./cm. The resulting mixture was continuously supplied into a centrifugal separator having a centrifugal effect of 100 G at a flow rate necessary to sustain a retention time of 1.5 minutes to conduct solid-liquid separation, whereby a gelled dense emulsion in the state of that produced by Example 1 was obtained. By adding the emulsion together with gelatin into warm water and ripening as in Example 1, a photographic gelatino silver halide emulsion having a specific electric conductivity of 1,700 ,uv./cm. was obtained. The emulsion showed the same properties as a photographic gelatino silver halide emulsion prepared by a conventional process. On the other hand, the photographic silver halide emulsion which was prepared in the same manner except that water was not added to the emulsion had a specific electric conductivity of 2,500 av./ cm.

EXAMPLE 4 A silver chlorobromide emulsion containing 30 parts by weight of silver bromide, 60 parts by weight of silver chloride and 5 parts by weight of dried gelatin per 1000 parts by weight of the total emulsion was mixed with an aggregating agent and an acid as in Example 1. The thus obtained silver halide emulsion having a specific electric conductivity of 75,000 ,uv./ cm. and in a state of becoming aggregated with the silver halide and the gelatin was continuously mixed with washing water in a mixing ratio of 1 to 3.3 as in Example 1 but for varying the pH of the emulsion and the water as set out in the table below by changing the initial pH of the emulsion at the initiation of aggregation (flocculation). The pH of the wash water was the. same as the pH of the emulsion prior to wash water addition so the wash water did not cause the pH to vary. The resulting mixture was then continuously supplied into a centrifugal separator having a centrifugal effect of G at a flow rate necessary to sustain a retention time of 1.5 minutes to conduct solid-liquid separation, whereby a gelled dense emulsion was obtained. By adding the emul- Treated emulsion Specific elec- Adjusted pH of emulsion trio eonductiv- Specific and water ity m/em.) gravity In Run No. 1 the aggregated emulsion was so sticky it attached strongly to the wall of the separator and could not be recovered completely from the separator. The specific gravity values show that about of the silver halide was lost.

In Run No. 5, aggregation of the emulsion was insufficient and the silver halide could not be separated completely by centrifuging. About of the silver halide tflowed away with the supernatant liquor.

The results of the above experimentation show the importance of adjusting the pH to within the range of 3.0 to 4.2 to obtain the results of the present invention.

EXAMPLE 5 Following Run 3 of Example 4, but varying the amount of water added to the emulsion before centrifuging, the effect on the residual salt content in the cinema: after only one centrifuging operation was analyzed.

FIG. 1 shows the specific electric conductivity of the mixture of emulsion and water vs. the ratio of water to the emulsion; and

FIG. 2 shows the specific electrical conductivity of the treated emulsion vs. the ratio of water to emulsion.

As is apparent from the Examples 1-3, in the case of mixing the emulsion with no water before the centrifugal separation, the electric conductivity of the thus obtained emulsion is higher than that of the emulsion to which water is added. That is, in order to reduce the electric conductivity to the desired level such as 1,700 ,av./cm., the centrifugally separated emulsion must be redispersed in water, followed by the same procedure. Accordingly, two procedures the same as the above must be repeated. In accordance with the present invention, however, one obtains a product which can be easily, directly redispersed without the prior art requirements of multiple repetitions of fiocculating, dispersing, etc.

What is claimed is:

1. In a process for fiocculating a physically ripened photographic gelatino silver halide emulsion by the addition of a fiocculating agent thereto the improvement which comprises the steps consisting of:

(a) adding to said emulsion wash water having a pH such that the pH of said emulsion after wash water addition is within the range of pH 3.0 to 4.2, the addition of wash water occurring no later than the initiation of flocculation of said emulsion, said fiocculation being initiated at a pH within the range of 3.0 to 4.2;

(b) subjecting said emulsion to a centrifugal separation at a gravity force within the range of from 10 G to 1000 G at a temperature between 5 C. to 25 C. whereby said gelatino silver halide emulsion is flocculated, desalted and dehydrated simultaneously; and

(c) thereafter removing said emulsion in a condition suitable for redispersion.

2. The process of claim 1 where the pH of the gelatino silver halide emulsion is adjusted to be within the range 3.0 to 4.2 prior to the adding of said wash water.

3. The process of claim 2 wherein the pH of the wash water is substantially identical to the pH of the emulsion.

4. The process of claim 3 where the gravity force is in the range 10 G to 500 G.

5. The process of claim 3 further comprising redispersing said emulsion as a final step.

6. The process of claim 3 where the fiocculating agent is an inorganic salt.

7. The process of claim 3 where the fiocculating agent is a high molecular weight organic polymer which forms a hydrophobic complex with the gelatin.

8. The process of claim 3 where the gelatin concentration of the emulsion before the addition of the flocculating agent is less than 4 wt. percent.

9. The process of claim 3 where the ratio of the wash water to the emulsion is less than 10 volumes of wash water per volume of emulsion.

10. The process of claim 2 wherein the pH of said emulsion is adjusted by adding thereto an acid.

11. The process of claim 1 wherein the pH of said emulsion is adjusted to be within the range of 3.0 to 4.2 by the addition of said wash water.

References Cited UNITED STATES PATENTS 2,463,794 3/1949 Murray 96-114 3,007,796 11/1961 Steigmann 961l4.7 3,436,220 4/ 1969 Dersch 96114.8

FOREIGN PATENTS 157,218 12/1963 Russia 9694 139,192 1961 Russia 96-94 649,546 1/ 1951 Great Britain 96114.8 965,148 7/1964 Great Britain 96114.7

J. TRAVIS BROWN, Primary Examiner M. F. KELLEY, Assistant Examiner US. Cl. X.R. 96-94 R, 114.7 

